Testing and evaluation of microwave systems often requires the availability of a high frequency test signal. Such test signals are commonly generated using so-called “comb” generators that are configured to receive a periodic input signal and produce an output signal that can have appreciable signal power at multiple harmonics of the input signal. This output signal typically consists of a series of uniformly spaced frequency components (“pickets”), so that the output signal spectrum has an appearance like that of teeth on a comb. Conventional comb generators produce harmonics using step recovery diodes (SRDs) to generate electrical signals that are intended to approximate impulses. A bandpass filter can be provided to select one picket from the comb of frequency pickets produced by the comb generator.
Unfortunately, conventional comb generators generally accept input signals over a narrow range of frequencies and power levels, thereby limiting user selection of picket spacing and frequency span. The limited output picket spacing can make selection of a particular picket from the comb difficult. In addition, due to the manner in which SRDs produce impulse-like signals, the use of conventional comb generators can introduce substantial phase noise.
In view of these and other limitations of conventional SRD based comb generators, improved comb generators and other methods and apparatus for generating periodic high frequency signals are needed.